The principle of ink jet recording system is to eject a liquid or fused solid ink from nozzle, slit, porous film or the like to make recording on a recording material such as paper, cloth or film. For ejecting an ink, various methods have been proposed, such as a method of ejecting an ink using an electrostatic induction, namely, so-called charge control system; a method of ejecting an ink using a piezoelectric element and an oscillation pressure; and a method of ejecting an ink using a pressure generated as a result of forming and growing bubbles by heat, so-called thermal ink jet system. Using these methods, a high precision image can be obtained.
The ink for ink jet recording is required to have the following properties:
(1) that a high-resolution and high-density uniform image can be obtained without causing blotting or fogging on paper; PA1 (2) that clogging due to drying of the ink does not occur at a nozzle tip and ejection responsibility and ejection stability are always kept in good condition; PA1 (3) that the ink on paper exhibits good drying property; PA1 (4) that the image has good fastness; and
(5) that high stability is ensured in the long-term storage.
As the ink for ink jet recording, an ink comprising a dye as a coloring material is predominantly used. However, the dye is disadvantageous in that the image is inferior in the water resistance and light fastness. To cope with this, development of an ink using a pigment as a coloring material is proceeding. Since the pigment is substantially insoluble in water, studies are being made on the method using a dispersant for stably dispersing the pigment in ink. However, if a surfactant is used as the dispersant, the surface tension excessively decreases and troubles are readily caused, such as deterioration in the quality of the printed image and printing drop out due to bubbles. Accordingly, use of a dispersant comprising a polymer compound is being studied.
As the polymer dispersant, various copolymers having both a hydrophobic moiety highly affinitive for a pigment and a hydrophilic moiety highly affinitive for an aqueous medium have been proposed. For example, an ink containing a polymer dispersant and a specific solvent (JP-A-56-147859 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), an ink containing acidic carbon black and a water-soluble resin having a specific molecular weight in a specific content ratio (JP-A-3-210373) and an ink containing acidic carbon black and a water-soluble resin having a specific acid value (JP-A-4-18461) are disclosed.
With respect to the form of the copolymer for the polymer dispersant, inks using a random copolymer (JP-A-56-147859), a block copolymer (U.S. Pat. No. 5,085,698) or a graft copolymer (JP-A-6-100810) are disclosed. Block copolymers and graft copolymers controlled in the orientation of monomers are considered to have an ideal structure as the dispersant, however, these are disadvantageously complicated in the synthesis process and in turn expensive. Conventional random copolymers are relatively easy to produce, however, properties of this polymer such as the kind of monomer selected, molecular weight and acid value, have not yet been thoroughly studied and copolymers satisfying all of the above-described properties required have not yet obtained.